In a chain transmission device configured to transmit the rotation of a crankshaft to camshafts through a timing belt or a timing chain (hereinafter, simply referred to as “chain”), a pivotable chain guide is brought into contact with the loose side of the chain, and an adjustment force is applied to the chain guide by a chain tensioner so as to keep the tension of the chain constant.
As such a chain tensioner, a chain tensioner is known which includes a housing in which a cylinder chamber is formed, a plunger mounted in the cylinder chamber, and a return spring biasing the plunger outwardly, wherein the housing is formed with an oil supply passage communicating with a pressure chamber formed rearwardly of the plunger, wherein the chain tensioner further includes a check valve mounted at the oil outlet side of the oil supply passage so as to prevent hydraulic oil in the pressure chamber from flowing back into the oil supply passage, and wherein the hydraulic oil supplied into the pressure chamber through the oil supply passage buffers the inwardly pressing force applied to the plunger from a chain.
In such a chain tensioner, when the engine is activated, air may be mixed into hydraulic oil being supplied into the pressure chamber through the oil supply passage from an oil pump, and could reach the pressure chamber.
Also, when the plunger moves outwardly rapidly, and the pressure in the pressure chamber decreases rapidly, air might go into the pressure chamber from the outside of the chain tensioner through between the sliding surfaces of the cylinder chamber and the plunger.
If air remains in the pressure chamber, when the chain becomes tense, and thus the plunger is pressed inwardly, the plunger compresses the air, so that the damper effect deteriorates. Therefore, the chain tensioner is unable to function fully.
In order to overcome such a problem, the below-identified Japanese Patent Application Publication No. H10-26192 discloses a chain tensioner further including a screw threadedly engaged in a threaded hole extending to the pressure chamber from the outer periphery of the upper portion of the housing, and a spring washer mounted between the head of the screw and the outer periphery of the housing, and having circumferentially separate radial ends wherein the pressure chamber communicates with the outside of the chain tensioner through a helical gap defined between the threadedly engaged portions of the screw and the threaded hole, and through between the separate radial ends of the spring washer, and wherein when the plunger is pressed inwardly so that the pressure in the pressure chamber increases, the air remaining in the pressure chamber can be discharged to the outside through a communication passage comprising the helical gap and the separate radial ends of the spring washer.
In the chain tensioner disclosed in Japanese Patent Application Publication No. H10-26192, the amount of air discharged from the pressure chamber is controlled by the diameter size of a prepared hole for the threaded hole or by the tightening torque of the screw. However, if the tightening torque is larger than a predetermined value or if foreign matter mixed in engine oil (hydraulic oil) such as dust clog the space between the circumferentially separate radial ends of the spring washer, air is less likely to be discharged from the pressure chamber, so that the air remaining in the pressure chamber tends to hinder the chain tensioner from functioning properly. Therefore, it was necessary to take a measure to reliably discharge air from the pressure chamber.
It is an object of the present invention to provide a chain tensioner in which the air remaining in a pressure chamber can be smoothly and reliably discharged to the outside of the chain tensioner, even if the tightening torque of a screw is not controlled fully, or even if foreign matter clog the space between separate ends of a spring washer for preventing the screw from loosening.